Because of the various excellent advantages, zinciferous plated steel sheets are widely applied as various rust-preventive steel sheets. In order to use these zinciferous plated steel sheets as rust-preventive steel sheets for automobile, it is important for these steel sheets to be excellent in press-formability, spot-weldability, adhesiveness and chemical treatability as properties required in the manufacturing step of automobile bodies, in addition to corrosion resistance and painting adaptability.
The zinciferous plated steel sheet is however defective in that it is low in press-formability in general as compared with a cold-rolled steel sheet. This is attributable to a large sliding resistance between the zinciferous plated steel sheet and a die of a press as compared with sliding resistance between the cold-rolled steel sheet and a die of a press. More specifically, since the zinciferous plated steel sheet has a large sliding resistance, it becomes difficult for the zinciferous plated steel sheet to flow into the die of the press at a portion where sliding resistance between a bead of the die of the press and the zinciferous plated steel sheet is very large, and fracture tends to easily occur in the zinciferous plated steel sheet.
A method which comprises applying a high-viscosity lubricant oil onto the surface of a zinciferous plated steel sheet is commonly used as a method for improving press-formability of the zinciferous plated steel sheet. This method has however drawbacks in that the high-viscosity of the lubricant oil causes a defective degreasing, which results in a painting defect in the painting step, and lack of the lubricant oil during the press-forming causes unstable press-formability. There is therefore a strong demand for improvement of press-formability of the zinciferous plated steel sheet.
During the spot-welding of the zinciferous plated steel sheet, on the other hand, a copper electrode reacts with molten zinc, and this tends to form a brittle alloy phase. A problem in the zinciferous plated steel sheet is therefore that wear of the copper electrode is serious, leading to a short service life thereof, and as a result, the zinciferous plated steel sheet is poorer in continuous spot-weldability than the cold-rolled steel sheet.
In the manufacturing step of an automobile body, furthermore, various adhesive agents are used for the purposes of rust prevention and inhibition of vibration of the automobile body. Recently, however, adhesiveness of the zinciferous plated steel sheet has been found to be lower than that of the cold-rolled steel sheet.
As a method for solving the above-mentioned problems, Japanese Patent Provisional Publication No. 53-60,332 published on May 30, 1978 and Japanese Patent Provisional Publication No. 2-190,483 published on Jul. 26, 1990 disclose a technology for improving weldability or workability of a zinciferous plated steel sheet, which comprises subjecting the zinciferous plated steel sheet to an electrolytic treatment, a dipping treatment, an application/oxidation treatment or a heating treatment, thereby forming an oxide film mainly comprising zinc oxide (ZnO) on the surface of the zinciferous plated steel sheet (hereinafter referred to as the "prior art 1").
Japanese Patent Provisional Publication No. 4-88,196 published on Mar. 23, 1992 discloses a technology for improving press-formability and chemical treatability of a zinciferous plated steel sheet, which comprises dipping the zinciferous plated steel sheet in an aqueous solution containing sodium phosphate in an amount within a range of from 5 to 60 g/l and having a pH value within a range of from 2 to 6, or spraying said aqueous solution onto the surface of the zinciferous plated steel sheet, or subjecting the zinciferous plated steel sheet to an electrolytic treatment in said aqueous solution, thereby forming an oxide film mainly comprising phosphorus oxide on the surface of the zinciferous plated steel sheet (hereinafter referred to as the "prior art 2").
Japanese Patent Provisional Publication No. 3-191,093 published on Aug. 21, 1991 discloses a technology for improving press-formability and chemical treatability of a zinciferous plated steel sheet, which comprises subjecting the zinciferous plated steel sheet to an electrolytic treatment, a dipping treatment, an application treatment, an application/oxidation treatment or a heating treatment, thereby forming a nickel oxide film on the surface of the zinciferous plated steel sheet (hereinafter referred to as the "prior art 3").
Japanese Patent Provisional Publication No. 5-867,885 published on Apr. 22, 1983 discloses a technology for improving corrosion resistance of a zinciferous plated steel sheet, which comprises subjecting the zinciferous plated steel sheet, for example, to an electroplating treatment or a chemical plating treatment, thereby forming a metallic film of nickel and/or iron on the surface of the zinciferous plated steel sheet (hereinafter referred to as the "prior art 4").
Japanese Patent Provisional Publication No. 3-17,282 published on Jan. 25, 1991 discloses a method for causing the substitution/precipitation of at least one metal selected from the group consisting of iron, nickel and cobalt on the surface of a zinciferous plated steel sheet (hereinafter referred to as the "prior art 5").
Japanese Patent Provisional Publication No. 6-063,394 published on Apr. 11, 1985 discloses a method for applying an aqueous solution containing ingredients of an inert film on the surface of a zinciferous plated steel sheet (hereinafter referred to as the "prior art 6").
The above-mentioned prior arts 1 to 6 have however the following problems:
(1) In the prior art 1, since an oxide film mainly comprising zinc oxide (ZnO) is formed on the surface of the zinciferous plating layer by any of the various treatments as described above, ordinary weldability, i.e., joinability between welded sheets, and workability except for press-formability of the zinciferous plated steel sheet, are improved, whereas the reducing effect of sliding resistance between the die of the press and the zinciferous plated steel sheet is slight. It is therefore difficult to improve press-formability of the zinciferous plated steel sheet, and the existence of an oxide film mainly comprising zinc oxide on the surface of the zinciferous plating layer, causes degradation of adhesiveness of the zinciferous plated steel sheet. PA1 (2) In the prior art 2, an oxide film mainly comprising phosphorus oxide is formed on the surface of the zinciferous plating layer. Therefore, while press-formability and chemical treatability of the zinciferous plated steel sheet are improved, spot-weldability and adhesiveness thereof are degraded. PA1 (3) In the prior art 3, a film of a single phase of nickel oxide is formed on the surface of the zinciferous plating layer. While, therefore, press-formability of the zinciferous plated steel sheet is improved, adhesiveness thereof is degraded. PA1 (4) In the prior art 4, a metallic film of nickel or the like is formed on the surface of the zinciferous plating layer. This improves corrosion resistance of the zinciferous plated steel sheet. However, because of the strong metal properties of the film described above, there is unavailable a sufficient improving effect of press-formability and spot-weldability of the zinciferous plated steel sheet. In addition, the prior art 4 poses another problem in that a low wettability of the metal to an adhesive agent makes it difficult to obtain a sufficient adhesiveness of the zinciferous plated steel sheet. PA1 (5) In the prior art 5, a metallic film substitution-precipitated on the surface of the zinciferous plated steel sheet has only a low wettability to an adhesive agent, thus making it difficult to obtain a sufficient adhesiveness of the zinciferous plated steel sheet. Because of the strong metal properties of the film described above, there is available only a slight improving effect of press-formability and spot-weldability of the zinciferous plated steel sheet. The aqueous solution for forming the metallic film has a low pH value and a low efficiency of substitution/precipitation. A sufficient quantity of deposited metal cannot therefore be ensured. In order to ensure a sufficient quantity of deposited metal, therefore, it is necessary to increase a temperature of the aqueous solution, resulting in such problems as an increased manufacturing cost including an increased consumption of energy and an installation of heating facilities of the aqueous solution. PA1 (6) In the prior art 6, an inert film is formed on the surface of the zinciferous plated steel sheet. Chemical treatability and adhesiveness of the zinciferous plated steel sheet are degraded as a result. PA1 a steel sheet, at least one zinciferous plating layer formed on at least one surface of said steel sheet, and an Fe--Ni--O film as an uppermost layer formed on said at least one zinciferous plating layer; PA1 a total quantity of metallic elements contained in said Fe--Ni--O film being within a range of from 10 to 1,500 mg/M.sup.2 ; and PA1 an oxygen content in said Fe--Ni--O film being within a range of from 0.5 to under 30 wt. % (hereinafter referred to as the "zinciferous plated steel sheet No.1 of the present invention"). PA1 the ratio of the iron content (wt. %) relative to the total quantity of the iron content (wt. %) and the nickel content (wt. %) in said Fe--Ni--O film being within a range of from over 0 to 0.9 (hereinafter referred to as the "zinciferous plated steel sheet No. 2 of the present invention"). PA1 the ratio of the iron content (wt. %) relative to the total quantity of the iron content (wt. %) and the nickel content (wt. %) in said Fe--Ni--O film being within a range of from 0.05 to under 1.0 PA1 the ratio of the iron content (wt. %) relative to the total quantity of the iron content (wt. %) and the nickel content (wt. %) in said Fe--Ni--O film being within a range of from 0.05 to 0.9, and the oxygen content in said Fe--Ni--O film being within a range of from 0.5 to 10 wt. % PA1 the total quantity of said metallic elements contained in said Fe--Ni--O film being within a range of from 10 to 1,200 mg/m.sup.2, and the ratio of the iron content (wt. %) relative to the total quantity of the iron content (wt. %) and the nickel content (wt. %) in said Fe--Ni--O film being within a range of from 0.1 to 0.3 PA1 a ratio of an iron content (g/l) relative to a total quantity of the iron content (g/l) and a nickel content (g/l) in said aqueous solution being limited within a range therefrom over 0 to 0.9 PA1 the ratio of the iron content (g/l) relative to the total quantity of the iron content (g/l) and the nickel content (g/l) in said aqueous solution being limited within a range of from 0.05 to under 1.0 PA1 the ratio of the iron content (g/l) relative to the total quantity of the iron content (g/l) and the nickel content (g/l) in said aqueous solution being limited within a range of from 0.05 to 0.9 PA1 the ratio of the iron content (g/l) relative to the total quantity of the iron content (g/l) and the nickel content (g/l) in said aqueous solution being limited within a range of from 0.1 to 0.3
An object of the present invention is therefore to solve the above-mentioned problems involved in the prior arts 1 to 6, and to provide a zinciferous plated steel sheet, particularly a zinciferous plated steel sheet which is excellent in press-formability, and furthermore, is excellent in at least one of spot-weldability, adhesiveness and chemical treatability, depending upon the use.
Another object of the present invention is to solve the above-mentioned problems involved in the prior arts 1, 3, 5 and 6, and to provide a method for manufacturing a zinciferous plated steel sheet, particularly a zinciferous plated steel sheet which is excellent in press-formability, and furthermore, is excellent in at least one of spot-weldability, adhesiveness and chemical treatability, depending upon the use.